Who is the best champion to use for mid?
As of now, I use Fiddlesticks or Shyvana for mid, and Talon or Zed for bot. The only problem with Fiddle is that he runs out of mana eventually, so I'll be stuck at mid with only basic attacks against a player without mana. Is there anyone better? Or should I switch lanes entirely?

And WHAT DO I USE FOR AN ITEM SET FOR FIDDLE BECAUSE I'M NOT DOING TO GREAT WITH THAT xD
Last thing, I was thinking about getting Karthus, is he a good champion? His passive and Ult are great, I personally think. How much is he in RP/IP...

works well.
You can also work towards an Athene's Unholy Grail or another Mana Regen item on most mana champions if you need the extra mana regeneration. However, learning to conserve is always best.